Carbon fibre
The term carbon fibre
is referred to those fibres that have been heated to temperatures up to 1500°C
a contains up to 95% of elemental carbon, Another type of carbon fibre is
commonly known as 'graphite' fibre. Graphite fibres are those which have been
heated to higher temperatures, often above 2500°C, and are about 99% carbon.
Such fibres have broader graphite-like layers, which are closely packed in a
parallel alignment.
Carbon fibre formation
Formation of carbon
fibre generally consists of the following stages:
(a)
Conversion of the precursor material into a fibre, if it is not in fibre form.
(b)
Elimination of all chemicals other than carbon by thermal cleavage, oxidation,
etc.
(c)
Carbonization for conversion into carbon fibre.
(d)
Graphitisation for conversion into graphite fibre.
Precursor Material
Carbon fibres are
generally manufactured by pyrolysis and thermal treatment of organic precursor
fibres like rayon, polyacrylonitrile, pitch, coal tar. Several temporally
stable polymers and/or fibres can be used as a precursor material for conversion
into carbon fibre. These fibre include poly (vinyl alcohol), polyimide,
aromatic polyamide, and polybenzimidazole. The main characteristics of the
precursor material for the conversion are that the melting temperature should
be substantially higher than the decomposition temperature.
Carbon fibre from rayon
Cellulose or rayon is
one of the most widely used precursors or starting material for making carbon
fibre. Rayon yields 15-30% by weight of carbon and does not melt during
decomposition. So the physical form of the starting material can be maintained.
The conversion generally takes place in the furnace at different temperatures
and at different heating rates. The heating takes place in different stages
like :
(a) In the first stage
of heating, the temperature should rise from 10°C/hr to 50°C/hr in the temperature
range 100° - 400°C. The temperature range 250° - 300°C is very critical as the
maximum weight loss takes place in this region.
(b) In the second stage
of heating, the temperature should rise from 50°C/hr to 100 C/hr in the range
400 - 900°C.
(c) In the third stage
of heating, the fibre is heated to 3000°C until graphitization takes place.
The multistage mechanism
for the conversion of cellulose to carbon in outline is as follows:
Stage II:
Physical desorption of water in the temperature range of 25°. 150°C.
Stage Il :
Dehydration from the cellulose unit at 150°C- 240°C
Stage III :
Thermal cleavage of the cyclotide linkage and scission of other C-O bonds and
to some C-C bonds in the temperature range of 240°C - 400°C via a free radical
reaction.
Stage IV :
Aromatization at or above 400°C.
Carbon fibre from pan
Polyacrylonitrile (PAN)
is used as a starting material to manufacture carbon fibre. The denier of PAN
fibre should be 1 to 3. Different stages to convert PAN fiber to carbon fibre
are as follows: Oxidation The fibre is oxidized at 200 - 250°C in air for
sufficient time. The fire may be kept in stretch/tension conditions during
oxidation. After oxidation, the fibre is black and has a shiny appearance. Carbonizations
The fibre is further heat-treated in an inert condition and in the temperature
range of 800°C to 1000°C for at least one I hr.
The fibre must be kept
out of contact with air for which nitrogen gas should be used during the reaction.
Heat treatment or graphitization The fibre is further heat-treated in the
temperature region of 1100°C - 2500 C. Heat treatment at 1100°C- 1500°C would
yield a lower modulus but high strength fibre. If the fibre is stretched during the oxidation or carbonization stage, the fibre will be an ultra high strength
fibre.
Graphitization at 2800°C would yield a graphite fibre, which is an oriented high modulus fibre. The various mechanisms for the conversion of PAN to carbon fiber are listed below: Oxidation induces a chemical reaction with the formation of oxygen bridge - linking two PAN molecules and water is eliminated. Stage I: Stage II: Carbonisation will form a carbon ring structure by carbonizing oxidized PAN fibre with the elimination of water and hydrocyanic acid. Stage III: Further heat treatment or graphitization modifies the structure of the fibre to an oriented fibrillar structure.
Carbon fibre from the pitch
Petroleum and coal-derived pitches are the basic raw materials for the manufacture of carbon and
graphite fibre. Pitch is a complex mixture of aromatic hydrocarbon molecules of
wide molecular weight distribution. It. contains more than 90% carbon, much
more than rayon or PAN.
Initially. low-modulus
carbon fibre from the pitch was manufactured. In this process. pitch is melted and
the thermoset by heating in o7one and/or air. This fibre is basically a low
modulus fibre.
High modulus carbon
fibre from the pitch is manufactured by converting the pitch to a mesophase or
liquid crystal pitch. This mesophase pitch can be melt spun and drawn into fine
filaments with high orientation. The conversion of the pitch into high modulus
carbon fibre consists of the following stage
a)
Polycondensation of the pitch for 2-8 hrs at 350°C - 450°C or hydrogenated with
selected chemicals at 360°C - 430°C for 2-6 hrs.
b)
Purification and heat treatment of pitch either for 10-15m at 500°C or for me hr
to 8 hr at 350°C - 450°C.
c) Melt
spinning into fibre at 290°C - 330°C.
d)
Oxidation for 20 m to I hr at 250°C - 600C.
e)
Carbonization for 10m - 30m at 1400°C - 1500°C.
f) Graphitisation
for 5 min at 2500°C - 2700°C to manufacture graphite fibre.
Properties of carbon fibre
The physical properties
of different types of carbon fibre are shown in Table 17.5. The fibre does not
melt. It oxidizes very slowly in the air at temperatures above 330°C. The fiber
exhibits excellent resistance to acids, alkalies even at high concentrations and
temperatures. It is also inert to all solvents. However, strong oxidizing agents
will degrade carbon fibre. Also, the fiber has poor resistance to hydro chlorites.
The fibre content is dyed. Physical properties of different carbon fibre
Application of carbon fibre
Carbon fibre was first
used as a light bulb filament in 1879. Diversifications in its application
started in 1944. From 1944 to 1964 low modulus carbon fibre was used as
thermal insulating material as well as in electrical insulation. After the invention of high strength and high modulus carbon fibres from PAN and pitch,
the fibre is used in high-performance composites in particular as rigid lightweight and dimensional stable reinforced material for aircraft and rockets. The
fiber is marketed as Celion, Hi-Tex, and Thrnel.
